parallels: fix libvirt crash if parallelsNetworkOpen fails
    
    If, by any reason, parallelsNetworkOpen fails it dereferences
    newly allocated privconn->networks via virObjectUnref, which in
    turn deallocates its memory.
    Subsequent call of parallelsNetworkClose calls virObjectUnref
    that leads to double memory free. To prevent this we should zero
    privconn->networks to make all subsequent virObjectUnref be safe.
    
    Signed-off-by: Maxim Nestratov <mnestratov@parallels.com>